UPDATE: In the latest Firefox 4 beta update, Mozilla is officially branding Firefox’s visual tab organization feature as Panorama.
Latest Firefox 4 nightly builds now feature Tab Sets, the ability to organize your tabs by visually grouping them to match your browsing style, introduced a few weeks ago as TabCandy.
If you have tried the experimental build released at that time, you won’t notice much change except some important bug fixing for improved stability.
A new tab sets button is placed next to the list all tabs button to show your tab sets. From there just draw a box to create a new empty group, drag a tab between sets, drag a tab onto another to create a new group, resize groups and tabs, and so on.
It should be available with the next forthcoming Firefox 4 Beta 4, later this month.